Blunted thermogenic responses to such stimuli as food and exercise may be one factor underlying obesity. Further understanding of the role of inadequate energy expenditure and subtle metabolic defects in obesity may lead to more effective treatment strategies. The thermic effect of food at rest and during exercise has been studied but the effect of exercise prior to a meal has not been examined systematically and the comparison the thermic effect of food ingested prior to or after exercise has not been made, nor have the interactions among the sequence of the meal and exercise, meal size, and work intensity been studied and compared in lean and obese subjects. The study of thermogenic responses to the confluence of food and exercise will provide quantitative information about the calorigenic effect of exercise under everyday conditions and clarify the relationship between obesity and impaired thermogenesis. Ten lean (less than 25% body fat) women and ten obese (greater than 35% body fat) but otherwise healthy women with early onset obesity will be studied. Body composition will be measured by hydrostatic weighing and submaximal aerobic fitness will be estimated by graded exercise testing to determine the anaerobic threshold. The two groups will be matched with respect to age, height, and level of aerobic fitness. Oxygen consumption and associated variables will be measured over 3 hour periods in the postabsorptive and postprandial states at rest and after 30 min of high (above anaerobic threshold) and low (below anaerobic threshold) intensity bicycle exercise. Energy expenditure during the exercise periods will also be measured. Two sequences (meal first and exercise first) of food and exercise, two meal sizes (400 and 800 kcal), and two work intensities will be given. Control tests (no food, no exercise) will also be given. Each subject will receive every combination of food and exercise on separate days in randomized order. The data will be used to compare in lean and obese women the effect of prior exercise on the subsequent thermic effect of food; the effect of a meal on subsequent exercise and postexercise energy expenditure; and the differential effect on postexercise metabolic rate of a meal before or after exercise. Interactions between meal size and work intensity will also be evaluated. The hypothesis that thermogenic responses are significantly smaller for obese than lean women will be tested and the sources and magnitude of impaired thermogenesis in the obese will be identified and quantified.